First Aspect of Invention: Hot Stamping
The continuously increasing requirements in automotive industry regarding light weight construction and improvement of passenger safety can be achieved successfully by the hot stamping approach, where AlSi coated ultra-high strength steels (UHSS; 22MnB5) are nowadays commonly used for production of structural parts (e.g. b-pillar). The AlSi topcoat provides protection against blank oxidation during the heat treatment (austenitization) at about 930° C. in ambient atmosphere, enables enhanced lubrication between tool and metal blank surface during the forming process and finally acts as passive corrosion barrier once the component is part of the vehicle chassis. Unfortunately, the AlSi topcoat exhibits a strong tendency to build-up on the tool steel surface upon forming operation at high temperatures which leads to significant adhesive wear. In order to increase productivity by avoiding time-consuming tool maintenance periods, a high temperature stable PVD coating solution with low build-up tendency for AlSi is required by automotive industry. For hot stamping applications TiAlN and AlCrN coatings are currently used. These material systems are characterized by excellent high temperature properties like adhesion strength, hardness, structural stability and oxidation resistance. Unfortunately both material systems exhibit a significant AlSi build-up tendency.
Hot Stamping Coating Requirements According to First Aspect
High temperature stable PVD coating solution which prevents effectively AlSi build-up upon long-term operation
Long-term performance at T˜800° C.!!
Structural stability
Chemical stability
Oxidation resistance
Resistance against thermo-mechanical fatigue
Resistance against thermal shocking
Hot hardness
Abrasive wear resistance
Long-term resistance against AlSi build-up
It is an objective of the present invention to disclose a coating for stamping tools which provides for low abrasive and low adhesive wear during hot stamping of AlSi-coated metal blanks.
Second Aspect of Invention: Coated Warm or Hot Forming Tools with Enhanced Performance
The second aspect of the invention relates to a warm or hot forming tool coated with a hard coating comprising at least one a-C:H:W hard layer for attaining enhanced tool life time and performance.
State of the Art
In recent years hot stamping of ultra-high strength steel sheets (as e.g. 22MnB5) is used more and more for manufacturing of automobile components (as e.g. B-pillar) with reduced weight in order to decrease on the one hand CO2 emission but also to improve on the other hand simultaneously passenger safety. Thus structural components (as e.g. B-pillar) made of the ultra-high strength steel sheets can be made significantly lighter by simply reducing the sheet thickness. For these reasons, the application of hot stamping or hot sheet metal forming processes for manufacturing of new structural automobile components has increased considerably in recent years and therefore it is necessary to bring into focus the complications which have arisen in parallel as well.
In Europe for example a hot metal sheet forming method called die quenching, hot forming, hot stamping or hot pressing is employed for fabricating automobile structural components having a tensile strength of around 1500 MPa (after processing). By this method the pre-heated (up to about 950° C. for homogeneous austenitization) ultra-high strength steel sheets can be easily formed and hardened in one stroke upon closing the forming press. The process can be described as follows: A heated ultra-high strength steel sheet is extracted from a heating furnace, then transferred within a few seconds to a pressing machine, subsequently formed into a prescribed shape using dedicated hot metal sheet forming tools which are maintained at room temperature and thus by quenching the ultra-high strength steel sheets during forming it can be hardened by the phase transformation from austenite to martensite exhibiting finally a tensile strength of about 1500 MPa. The press is kept closed for several seconds until the martensite transformation is completed (Senuma, T.: ISIJ Int. 41, 520 (2001)).
Generally speaking, as the tensile strength of a steel sheet increases, its formability and ductility decreases. Therefore, to overcome these limitations, various types of ultra-high strength steel sheet products have been developed and are still a matter of on-going research (Senuma, T.: ISIJ Int. 41, 520 (2001); 8. Erlanger Workshop Warmblechumformung Nov. 12, 2013).
In order to protect the steel sheet surface from uncontrolled oxidation (i.e. scale formation) during heating up to about 950° C. in ambient atmosphere, protective top-coats are frequently employed as e.g. AlSi- or Zn-based coatings (J. Kondratiuk et al. Wear 270 (2011) 839). For this purpose aluminized steel sheets such as the so-called USIBOR 1500 (AlSi-coated) as well as different kinds of metal sheets coated with zinc based coatings have been developed. These metal sheet variants exhibit generally excellent hot-pressing properties and corrosion resistance quality.
However, in spite of the very promising properties exhibited by the coated (AlSi and Zn) metal sheet versions as mentioned above, there are serious process complications which can be described as follows: Both sheet coating materials (AlSi and Zn) exhibit at high temperatures very pronounced tendency to adhere (stick) onto the forming tool surface. After several successive forming cycles the adhered and accumulated material may result in scratches and eventually cracks (this problem is often called galling) on the surface of the formed products (e.g. B-pillar) and therefore may also result in reduced or unsatisfactory product quality. Furthermore, massive sticking of material onto the forming tool surface leads to frequent maintenance periods (cleaning of tool surface) in production environment which reduces productivity enormously. Moreover, oxidation of AlSi and Zn leads to formation of abrasive oxide phases. Thus, in direct contact with the tool surface and upon long-term operation abrasive wear becomes more and more relevant. Additionally, with particular relevance for Zn-coated metal sheets, the formation of micro-cracks upon forming is also of paramount significance with respect to corrosion performance.
In order to overcome these complications it has already been speculated that process lubrication might be useful to suppress galling for AlSi-coated and Zn-coated sheets but also to reduce the degree of micro-cracking, in particular for Zn-coated sheets. However, from an industrial point of view efficient process lubrication by solid or liquid agents is not possible as it would deteriorate massively the workshop environment and upon post-treatment of the formed parts unhealthy degreasing agents would be needed in order to remove the remnants of the lubricant from the surface of the formed parts.
In WO2012104048 it is mentioned that one concept for improving current performance by hot metal sheet forming processes using coated metal sheets is to apply a low friction/high wear resistant PVD coating on the hot metal sheet forming tool. Furthermore, WO2012104048 cited the results obtained by Clarysse et al (Clarysse, F. et al. Wear 264 (2008) 400-404) in the context of some investigations about the behavior of different coating systems in tests especially designed to assess the response of the coatings to galling. They reported that carbon-based composite layers such as DLC-type (DLC is a well-known abbreviation for referring to amorphous diamond like carbon coatings which can be also referred as a-C:H coatings if they contain beside carbon only hydrogen without any further elements) and WC/C (a WC/C coating is a a-C:H:W coating in the context of the present invention) perform outstanding regarding galling resistance and they recommended therefore to use this type of tool coatings instead of typical hard coatings like for example CrN, TiN and CrN/TiCrN for avoiding galling.
For better understanding of the state of the art, it is important to emphasize that the temperature at which Clarysse et al. performed the tests was not specified. However, according to the description of the experiments it is straightforward to conclude that the intention of Clarysse et al. was to investigate the behaviour of coatings with respect to cold metal sheet forming operations and not with respect to hot metal sheet forming operations.
The authors of WO2012104048 reported that the improved performance for cold metal sheet forming tools obtained by using the coatings proposed by Clarysse et al. is not given sufficiently for hot sheet metal forming processes of coated ultra-high strength metal sheets.
Furthermore, it is reported in WO2012104048 that when AlSi-coated ultra-high strength steel sheets like USIBOR 1500 are used, the galling phenomenon cannot be satisfactorily reduced and because of that galling wear continues being a problem. Additionally, the authors of WO2012104048 recommended using CrSiN coatings for avoiding galling of hot metal sheet forming tools.
Moreover, the use of nitriding and carbonitriding processes, as well as other kinds of surface treatments, such as plasma treatments and micro-structuring, is mentioned in WO2012104048 as an alternative for improving performance of hot metal sheet forming tools.
Likewise, in WO2011011129 it is mentioned that coatings which normally perform well in cold forming conditions tend to yield poor performance under warm and hot forming operations or under high contact loading conditions. The authors of WO2011011129 supposed that the low performance of the coatings in these situations can be attributed to the inability of the coating to withstand cyclic thermo-mechanical or high contact loading applications faced for example, in warm and hot forming applications. They explain that in warm and hot metal forming processes, the tooling is exposed to thermo-mechanical conditions and therefore experiences a high thermal gradient through the thickness of the tool for example. In addition, the surface of the tooling is also subjected to cyclic thermal loading and compressive-tensile stress cycles. Consequently, the thermo-mechanical load cycle of tooling in warm and hot forming operations is also significantly different than that of tooling in cold forming operations. Additionally, WO2011011129 proposed a coating which should be able to provide improved wear life as well as oxidation resistance properties for forming tools used in thermo-mechanical load applications comprising for example TiCxN(1-x) or TiMCxN(1-x), with M: Al or a transition element from Groups 4, 5 and 6 of the periodic table, as a bottom coating and having a top coating including for example alumina or aluminium containing phases.
Objective of the Invention
It is an objective of the present invention, to provide a warm or hot forming tool having improved lifetime and satisfactory performance in warm or hot forming operations, particularly in warm or hot metal sheet forming of coated metal sheets, in particular of metal sheets coated with AlSi- and Zn-based coatings.
Third Aspect of Invention: Coatings for Tribological Applications at Room and Elevated Temperatures
The third aspect of the invention relates to a hard coating comprising at least one Mo—C (molybdenum carbide) hard layer with dedicated architectural design for attaining enhanced tribological performance (low abrasive and adhesive wear) at room and elevated temperatures in contact with Al-, Zn- and Fe-based counter bodies.
State of the Art
The development and application of new technologies for industrial production (e.g. cutting or forming of steel for production of automotive body parts) as well as for subsequent consumer applications (e.g. use of automobiles for private issues) involves always unavoidably the necessity to be concerned with new and challenging tribological systems at room and elevated temperatures in contact with different counter body materials. Even though the development of new technologies might be theoretically possible, they can only be implemented practically in our daily live with success once the tribological systems involved are designed such that productivity and efficiency are not limited by wear. Against this background it follows straightforwardly that tribology and wear are of paramount importance for the industrial but also for our societal environment and thus the control of material wear is and will remain one major goal of our technological development. In particular at non-ambient temperatures it is mandatory to ensure the functionality of the surfaces (e.g. parts, components, tools, etc.) involved with respect to mechanical, structural and chemical stability. To this end hard coatings are often employed. However, it is still a great challenge to provide hard coatings with enhanced tribological properties for high temperature applications in order to reduce both simultaneously, abrasive and adhesive wear, but without losing mechanical, structural and chemical performance.
One proper example therefor is the “hot stamping” or “press hardening” technology developed some years ago with the goal to provide low-weight ultra-high strength steels (as e.g. 22MnB5) for manufacturing of automobiles with reduced CO2 emission. Thus structural components (as e.g. B-pillar) made of the ultra-high strength steel sheets can be made significantly lighter by simply reducing the sheet thickness but without losing mechanical performance which is important for passenger safety issues. For these reasons, the application of hot stamping processes for manufacturing of new structural automobile components has increased considerably in recent years, but on the other hand process complications like significant tool wear and other tribological phenomena arised in parallel as well which makes it currently absolutely mandatory to focus on these topics. In the following first a more detailed understanding of the hot stamping technology will be given before the process complications mentioned above can be described in the right context.
In Europe for example a hot metal sheet forming method called die quenching, hot forming, hot stamping or hot pressing is employed for fabricating automobile structural components having a tensile strength of around 1500 MPa (after processing). By this method the pre-heated (up to about 950° C. for homogeneous austenitization) ultra-high strength steel sheets can be easily formed and hardened in one stroke upon closing the forming press. The whole process can be described as follows: A heated ultra-high strength steel sheet is extracted from a heating furnace, then transferred within a few seconds to a pressing machine, subsequently formed into a prescribed shape using dedicated hot metal sheet forming tools which are maintained at room temperature and thus by quenching the ultra-high strength steel sheets during forming the steel sheet can be hardened by the phase transformation from austenite to martensite exhibiting finally a tensile strength of about 1500 MPa. The press is kept closed for several seconds until the martensite transformation is completed.
In order to protect the steel sheet surface from uncontrolled oxidation (i.e. scale formation) during heating up to about 950° C. in ambient atmosphere, protective top-coats are frequently employed as e.g. AlSi- or Zn-based coatings. For this purpose aluminized steel sheets such as the so-called USIBOR 1500 (AlSi-coated) as well as different kinds of metal sheets coated with Zn-based coatings have been developed. These metal sheet variants exhibit in general excellent hot stamping properties and good corrosion resistance.
However, in spite of the very promising properties exhibited by the coated (AlSi- and Zn-based) metal sheet versions as mentioned above, there are serious process complications (related to tool wear and other tribological phenomena at the tool/sheet interface) which can be described as follows:                Both sheet coating materials (AlSi and Zn) exhibit at high temperatures very pronounced tendency to adhere (stick) onto the forming tool surface; see also FIG. 17. After several successive forming cycles the adhered and accumulated material (this phenomena is often called galling) may result in scratches and eventually cracks on the surface of the formed products (e.g. B-pillar) and therefore may also result in reduced or unsatisfactory product quality.        Furthermore, massive sticking of (AlSi- or Zn-based) material onto the forming tool surface leads to frequent maintenance periods (cleaning of tool surface) in production environment which reduces productivity enormously.        Moreover, oxidation of AlSi- and Zn-based coatings leads to formation of abrasive oxide phases. Thus, in direct contact with the tool surface and upon long-term operation abrasive wear becomes more and more relevant.        Additionally (and with particular relevance for Zn-coated metal sheets) the formation of micro-cracks within the coating/substrate system upon forming is also of paramount significance with respect to corrosion performance; see also FIG. 17.        
In order to overcome these process complications occurring at the tool/sheet interface at high temperatures, it has already been speculated that process lubrication might be useful to suppress galling for AlSi- and Zn-coated sheets but also to reduce the degree of micro-cracking, in particular for Zn-coated sheets. However, from an industrial point of view efficient process lubrication by solid or liquid agents is not possible as it would deteriorate massively the workshop environment and upon post-treatment of the formed parts unhealthy degreasing agents would be needed in order to remove the remnants of the lubricant from the surface of the formed parts.
In WO2012104048 it is mentioned that one concept for improving tool performance during hot stamping of coated metal sheets is to apply generally a low friction/high wear resistant PVD coating on the forming tool.
From literature it is known that carbon-based composite layers such as DLC (DLC is a well-known abbreviation for amorphous diamond like carbon coatings also written as a-C:H if the coating contains beside carbon only hydrogen without any further elements) and WC/C (a WC/C coating is an a-C:H:W coating, i.e. a DLC coating with W) perform outstanding regarding galling resistance upon cold metal sheet forming operations and therefore it is recommended to use this type of tool coatings instead of typical hard coatings like for example CrN, TiN and CrN/TiCrN for avoiding galling.
However, sound statements including experimental results about performance of such carbon-based coatings at high temperatures are still not available. In WO2011011129 it is also only generally stated that coatings which normally perform good in cold forming operations tend to yield poor performance under hot forming operations and/or under high contact loading conditions. The authors supposed that this behavior is attributed to the inability of the coatings to withstand cyclic thermo-mechanical loading conditions during hot forming applications. Their proposal is to use for such applications TiCxN(1-x) or TiMCxN(1-x) (with M: Al or a transition element from groups 4, 5 and 6 of the periodic table) as a bottom coating and e.g. alumina or aluminium containing phases as top coating in order to account for oxidation and wear resistance as well.
A further concept to improve tool performance during hot stamping of coated metal sheets is the use of nitriding and carbonitriding processes and other kinds of surface treatments as well, such as plasma treatments and micro-structuring, as mentioned in WO2012104048. However, the authors recommended using CrSiN coatings for avoiding galling of hot metal sheet forming tools.
However, the application of the above mentioned tool surface concepts does still not provide sufficient improvement in tool performance during hot stamping of coated metal sheets. In particular for AlSi- and Zn-coated ultra-high strength steel sheets (e.g. 22MnB5) the galling phenomena and the micro-cracking issue (with particular relevance for Zn-coated 22MnB5) remain a problem of highest priority.
Objective of the Invention
Against the above background it is an objective of the present invention to provide a hard coating for attaining enhanced tribological performance (low abrasive and adhesive wear) at room and elevated temperatures in contact with Al-, Zn- and Fe-based counter bodies.